VEGETABLE PHYSIOLOGY. 11 



diversified by numerous tubes or intercellular openings 

 which act as conductors of the fluids whether essen- 

 tial or imbibed. These vessels having- generally a 

 longitudinal position, as if only intended for convey- 

 ing the sap directly upwards or downwards, and 

 though there are no very visible horizontally lying 

 tubes, it is notwithstanding quite evident there is a 

 lateral transfusion of the sap, probably by the diver- 

 gent or convergent partitions, and by intercellular 

 communication. 



These tubes are not always surcharged with sap, 

 some of them on this account have been called air 

 vessels. No doubt air itself, as well as fluids of a 

 less gross character, are necessary in the interior, as 

 well as to the exterior of plants. Electricity is 

 deemed one of the most effective agents in the pheno- 

 mena of vegetation ; and the spiral vessels found 

 on young shoots have been considered as the special 

 ducts for the conduction of electric currents. These 

 are circumstances, however, which are received and 

 credited more on the ground of their plausibility, 

 than resting on any practical proofs which have been 

 had of their reality. 



The bark, from its suberous character and station, 

 is well calculated for the conduction of fluids this, 

 the alburnum, and particularly in the space between 

 them in the spring, we see, are the principal channels 

 for the flow of the sap. The action of these tubular 

 organs is greatly assisted by the proper leaves and 

 other leafy expansions attached to the bark; even 

 the armature, it is probable, does a like office. 



